Differentiation, or what some have called the ‘negative starting point’ of integration, has always been the norm in EU defence policy. Driven by both endogenous and exogenous (f)actors, political leaders in the European Council are nevertheless mindful of the need for Member States to cooperate in more structured ways to better protect their citizens against security threats. For this reason, a package of harmonizing measures has been developed with remarkable speed since 2016. Permanent Structured Cooperation (PESCO) is the most prominent innovation in this field. Given the high levels of politicization in defence it is perhaps surprising that PESCO has produced the most inclusive expression of enhanced cooperation, even if it is the most flexible of the differentiated integration mechanisms provided by the Treaties. This is largely the result of a German push for inclusivity, which prevailed over a French desire for a higher level of ambition. Monitored by the European External Action Service (EEAS) and the European Defence Agency (EDA) and increasingly driven by the Commission’s Directorate General for Defence Industry and Space (DG DEFIS), which manages the European Defence Fund (EDF), PESCO is a force that generates ‘positive integration’ by de-fragmenting the defence market in the European Union. This article builds on empirical research that maps the varied clusters of Member States lining up behind different types of defence capability development projects. It observes a process of coagulation across the microcosm of PESCO, coupled with formal expressions of differentiated integration, both vertically and horizontally, and offers explanations for these trends. European Union, differentiated integration, defence, PESCO